Big Air on the Moon

February 14, 2006: "Go big or go home."
That's what aerialists on the US Olympic ski team say, and
when they say "big," they mean it.

Big
means "Big Air," 20 meters above the ground, as
high as a five-story building. Aerial skiers fly into the
void as fast as a motorcycle speeds down a city street, flipping
head over heels, twisting and flipping again. The sky tumbles,
but dizziness is not allowed, because only 3 heart-pumping
seconds after launch, it's time to land.

He
should know. In the sport of aerial skiing, Bergoust
has done it all. He was one of the first skiers ever to complete
a quad-twist triple flip--four twists and three flips in mid-air.
In 1998, hours after a frightening crash in practice, he used
the move to win gold at the Nagano Olympic Games. At the time,
his score was the highest ever recorded. This year, he's a
top contender again in Torino.

Above:
Eric Bergoust discusses skiing on the moon in an educational
video he made for NASA.

Bergoust
has a knack for invention. He has designed new skis to soften
the impact of practice landings in swimming pools. He has
altered the shape of ski jumps, called "kickers,"
to make flights longer-lasting and safer. And his take-off
method, raising one arm propeller-style to add twist to his
flight, is widely imitated.

His
next innovation: "We should jump on the moon! There's
plenty of fresh powder (moondust)," he explains. "And
I figure the 1/6 g would give us a lot of hang time."
More hang time means more flips--and more gold.

Consider
the following:

On
Earth, a typical run begins with Bergoust hurtling down a
23-degree slope. By the time he reaches bottom, 20 meters
below the starting gate, he's traveling almost 70 km/hr—directly
into the kicker. From a skier's point of view, the kicker
looks uncomfortably like a wall, but it's really a ramp guiding
the aerialist almost straight up in the air. Bergoust's favorite
kickers are angled at 70 degrees! Up he goes, hanging for
nearly 3 seconds before landing in soft snow another 20+ meters
beyond the ramp.

Now
imagine the same run—same hill, same kicker, same skier—on
the moon. Because lunar gravity is less, Bergoust would accelerate
downslope at a more gradual pace, reaching bottom with a speed
of only 28 km/hr. On Earth, such a slow start would be a disaster.
On the moon, it's perfect. Leaving the kicker at that speed,
Bergoust hangs in the "air" for a whopping seven
seconds, more than twice his hang-time on Earth: proof.

Right:
On the moon, Bergoust's aerial trajectory (red) would take
7+ seconds to complete. [More]

"I
might be able to double my quad-triple," he says.

Remember,
Bergoust won gold in 1998 with a quad-triple. Since then other
skiers have added a single twist to his move, turning it into
a quint-triple. "Quints" are expected to win the
men's freestyle aerials in Torino. On
the moon, Bergoust would have time to add four more
twists and three more flips to his routine. "Let's see…"
calculates Bergoust, "that would be an octuple-twist
sextuple flip." Guaranteed gold.

Now
for the problems:

Moondust,
although it is powdery, is not as slippery as snow. On the
contrary, moondust is very abrasive. It is made of tiny sharp
fragments of glass and rock produced by eons of meteoroids
pulverizing the moon. Compared to snow, moondust is a "slow
surface," maybe too slow for a good jump.

To
combat this, skiers are going to need extra-slick skis coated
with Teflon or some other low-friction material. Thin films
of diamond might be the answer. Diamond-like carbon films
in Earth laboratories rival Teflon in slipperiness, with the
advantage of diamond-like hardness to resist the scratching
action of sharp-edged dust.

Another
problem is the kicker. On Earth, kickers are made of snow.
Workers blow snow into large wooden forms laid out at the
base of the slope. A spray of water helps the snow stick together
and makes the ramp slippery-smooth. Disassemble the forms
and—voila!—a kicker.

Imagine
the same process on the moon. Workers assemble their form
and begin dumping moondust into it. There's no water hose
to squirt the dust to make it stick together. Water exposed
to lunar vacuum sublimates (vaporizes) in a flash. So the
dust remains dry. Disassemble the forms and—voila!—the kicker
slumps into a shapeless pile.

The
solution in this case might be a microwave water hose. In
labs on Earth, researchers have discovered that grains of
moondust cooked in a microwave oven quickly melt and stick
together. Spraying moondust with microwaves might allow Olympic
workers to mold a good kicker.

Above:
This snow-packed ramp, called a kicker, guides the aerialist
into the air. Problem: how would you build one on the moon?

And
finally—the landing.

On
Earth, aerial skiers land on a layer of soft snow, which cushions
their impact. On the moon, they'll land on a layer of soft
moondust. Very likely, the dust will spray upwards, coating
the skier's suit.

What's
the problem? Ask any Apollo astronaut. They hated it when
moondust got on their spacesuits. Dark dust absorbed sunlight,
causing the suit to overheat. Sharp edges of the dust cut
into seals, springing leaks. Dust-covered visors were hard
to see through. A skier's suit, thoroughly "dusted,"
might be useless after a single run. Another problem to solve....

Bergoust
loves solving problems. For years he's been tinkering with
skis, redesigning kickers, inventing new moves, and he's ready
for a new frontier.

Hey
wait a minute! The story tells us that Bergoust's
hang-time on the moon would be 7 seconds, more than
doubling his hang-time on Earth. Does he also fly higher
on the moon? Surprisingly, no. Contrary to what you
might expect, Bergoust's moon trajectory is the same
size and shape as his Earth trajectory. He would reach
the familiar height of 20 meters, and land the same
20+ meters downslope. The difference, Earth vs. moon,
is that on the moon he traces his arc more slowly, allowing
more time for twists and flips: proof.

Math
and physics teachers: you can read some of
the math behind this story here
and here.
See also Newtonian
Trajectories from Georgia State University's Hyperphysics
web site.

Jack
Skis the Moon -- (Science@NASA) The ski report is
out of this world: clear skies, no wind and deep powder.
Where is this fantastic resort? On the moon.

AirBergy.com
-- Eric Bergoust's home page, a good source of basic
information about aerial freestyle skiing.

Lunar
Olympics -- (Science@NASA) If winter Olympic Games
were held on the moon, where would they be? The lunar
Alps, of course.